Petruchi's thread -- Pool floor stains

[petruchi]

Hi folks! Thanks for the great site.

I look after the pool at a youth camp. We (for the first time in 7 years) have drained the pool for a thorough clean and to change out the pool lights. We had hoped to also clean up some odd stains on the pool floor around the water outlet vents. We have tried pretty well everything, with no effect. The only thing that has worked is scraping the floor with a knife at a very flat angle, which is laborious and painstakingly slow. I've attached some pics of the floor stains here:

http://imgur.com/a/3pI1I

The amazing thing to me is that a solution of muriatic acid has no effect on the stains whatsoever. Zilch. When the acid solution moves off the stain onto cleaner plaster, it begins to bubble and let off steam like you'd expect, and whitens it up nicely.

These stains have been on the pool floor for 6 years, pretty well since the pool had opened. There was a lot of construction in that first year, and the thought is that some fine construction dust had got into the system and stained the floor. Not sure what the pool chemistry was like at the time, but my guess is that it was still being worked out.

Any help? Thanks in advance for reading!

[PoolDoc]

Hi Petruchi;

I'm afraid you're not going to removed those stains, short of grinding them out, with a diamond disk. (And, I don't know if your plaster is thick enough to make that possible.)

If you look at the more closely, you can see that the shape is the shape of the water flow from the floor inlet:
https://picasaweb.google.com/lh/phot...eat=directlink

The unstained portions correspond to the 4 support bars that hold the center portion of the inlet.

And, in this edited view of the other picture,

https://picasaweb.google.com/lh/phot...eat=directlink

you can see that the plaster service is rough, and that the stain is in the depressions in the roughness.

What it looks like to me is you have (or have had) iron in the pool water, and have deeply stained the pool where the return water impinged directly on the plaster. You might be able to lighten it using "Iron Out", but you'd probably need a larger quantity than you can buy from the grocery store. They will sell direct in 25 - 50# quantities, so you could try a small bottle on a small area, and then buy a larger quantity if it worked.

Also, if the plaster is thick enough, you *could* remove the stains with straight acid, but you'd end up with a very rough and pitted surface even if you didn't burn though the plaster altogether.

PoolDoc

[petruchi]

Thanks for the reply!

We worked at it with a grinder with a 60 grit pad, and it does the trick. We have a 5/8" layer to work with, and the grinder does a pretty good job (with a steady hand!) at taking off the top layer. It almost appears that it's calcification, it is so much harder than the surrounding plaster.

We have had no success with chemicals at all, without getting drastic. Acid scares me, at least with the grinder we have some control.

Thanks again!

[PoolDoc]

Yeah.

I think, if you have 5/8" thick plaster, the grinder will give you the best and most durable results.

You might, if you feel like tinkering, use white epoxy paint or something to build up the outer lips of of your in-floor return, so that the water flow does not directly impact the plaster. Otherwise, if you still have iron in the water (steel tank sand filter, maybe?) the stains will return.

Ben

[petruchi]

Good advice, thanks. I think it was a problem of chemistry in that first year, as the stains never got any worse after the first year.

Appreciate the help!